Benchmark Oj::Parser in a thread safe way

The documentation state `Oj::Parser.usual` isn't thread safe:
https://github.com/ohler55/oj/blob/c70bf4125b546bc7146840b15de36460d42b4dff/ext/oj/parser.c#L1507-L1513

As such we shouldn't benchark it this way, but instantiate a new
parser every time. Technically in real world scenarios you could
create a pool of parsers and re-use them, but if it's not provided
by the gem, I'm not sure we should go out of our way to do it.

benchmark/parser.rb

@@ -19,7 +19,7 @@ def benchmark_parsing(name, json_output)
   Benchmark.ips do |x|
     x.report("json")      { JSON.parse(json_output) } if RUN[:json]
     x.report("oj")        { Oj.load(json_output) } if RUN[:oj]
-    x.report("Oj::Parser") { Oj::Parser.usual.parse(json_output) } if RUN[:oj]
+    x.report("Oj::Parser") { Oj::Parser.new(:usual).parse(json_output) } if RUN[:oj]
     x.report("rapidjson") { RapidJSON.parse(json_output) } if RUN[:rapidjson]
     x.compare!(order: :baseline)
   end